The present invention relates to an aromatic polycarbonate and in particular, to an aromatic polycarbonate which is improved in color tone and heat resistance.
Polycarbonate resins have been extensively used in various application fields because of excellent impact resistance, transparency or the like. Among others, these polycarbonate resins are especially useful as transparent materials such as optical discs, sheets, molding materials, alternate materials for glass products or the like. However, the polycarbonate resins have to be kneaded and molded at a high temperature of 250 to 380.degree. C. due to a high melting viscosity thereof. Therefore, there arises such a problem that these resins tend to be discolored when kneaded with additives and molded.
In order to solve the above problem, in the case where polycarbonate resins are produced by a so-called interfacial polymerization method of reacting an aromatic dihydroxy compound with phosgene, it has been attempted to improve the heat resistance of these resins by reducing an amount of methylene chloride used as a solvent, an amount of chlorine components derived from phosgene, or the like. On the other hand, in the transesterification method of heat-reacting the aromatic dihydroxy compound with the carbonic acid diester compound under a reduced pressure, it has been attempted to improve properties of the obtained polycarbonate resins by reducing a content of the carbonic acid diester as a raw material, deactivating the catalyst, etc. (Japanese Patent Application Laid-Open (KOKAI) No. 7-126374(1995)). However, these methods have failed to obtain polycarbonate resins having a sufficient thermal stability. Therefore, there has been demanded a further improvement in the production of polycarbonate.
As a result of the present inventors' earnest studies concerning the improvement of color tone, heat resistance and the like of polycarbonate resins for solving the above problems, it has been found that by reducing the content of a cyclic oligomer which is produced under specific conditions upon the production of polycarbonate resins, to not more than a specific value, and reducing the percentage of the cyclic oligomer content based on a total oligomer content, to not more than a specific percentage, the obtained polycarbonate resins can be considerably improved in color tone and heat resistance. The present invention has been attained on the basis of the finding.